<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .active {
        background-color: red;
      }
    </style>
  </head>
  <body>
    <div id="app">
      <ul>
        <li
          v-for="(item, index) in list"
          :class="{ active: index === activeIndex }"
          @click="activeIndex = index"
        >
          {{ item }}
        </li>
      </ul>
    </div>

    <script src="../../Vue.js"></script>
    <script>
      new Vue({
        el: "#app",
        data: {
          list: [111, 222, 333],
          activeIndex: 0,
        },
      });
    </script>
  </body>
</html>
